1. Field of the Invention
The present invention relates generally to position determination, and specifically to accurate position determination using reference signals with improved correlation characteristics.
2. Description of the Related Art
The Global Positioning System (GPS) technology has revolutionized the field of navigation and position location. Initially devised in 1974, the GPS is based on a constellation of 24 on-orbit satellites in sun-synchronous 12 hour circular, inclined orbits. Each satellite transmits a known pseudo-noise signal synchronized to an on-board precision atomic clock. The transmitted pseudo-noise signals can be precisely tracked by a receiver at an unknown location to determine pseudo-ranges. By tracking four or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in (1) B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996, and (2) J. Spilker, Jr., Digital Communications by Satellite, Prentice-Hall, Englewood Cliffs, N.J., 1977, 1995, which are incorporated by reference herein in their entirety.
However, the effectiveness of GPS is limited in some situations because the GPS signals are transmitted at relatively low power levels (less than 100 watts), and over great distances, the received signal strength is relatively weak (on the order of −160 dBW as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of line-of-sight blockage or while the receiver is inside a building.
This limitation or shortcoming of GPS technology can be overcome or ameliorated by employing position location technologies based on tracking broadcast television signals. High power, high bandwidth, low duty cycle, and superior geometries make various broadcast television signals ideal candidates for augmenting or improving position location where the GPS technology fails. For example, various digital television broadcast signals contain embedded synchronization codes which can be tracked for position determination purposes. The ATSC standard in the United States, the DVB standard in Europe, and the ISDB-T standard in Japan all employ an embedded synchronization code which is used to probe the transmission channel and mitigate the effects of multipath in a digital TV receiver. In order to be effective for channel modeling and multipath mitigation, these synchronization codes have wide bandwidths, narrow time autocorrelation functions, and high power levels. These features make the synchronization codes ideal for positioning, in particular for indoors positioning where multipath effects are severe and GPS signals may not penetrate. In addition, the Ghost-Canceling Reference (GCR) signals embedded in analog television broadcasts can be used for precise ranging. In recent years, analog television broadcasts have started to insert into their broadcasts a synchronization code termed the Ghost-Canceling Reference, which is used for multipath mitigation on analog signals in TV receivers that digitize the signal. High power characteristics and wide availability of GCR signals make them suitable for position location. Other test signals inserted in the analog broadcasts, such as the multiburst signal, may also be used for position determination.
In the GPS system, autocorrelation of a known Pseudo-Random-Number (PRN) sequence signal (i.e., a pseudo-noise signal) is used to determine pseudo-ranges. For synchronization code signals embedded in broadcast TV signals, however, autocorrelation of a synchronization code signal can yield significant sidelobes in addition to their main correlation peaks. This is because, unlike the GPS Gold-code PRN sequences, the signals were designed primarily for communication purposes, not for position determination. These sidelobes can be confused with multipath reflection signals, resulting in significant errors in pseudorange measurements. However, it is not necessary for the local reference signal to be an exact match of the broadcast synchronization signal to be suitable for pseudo-range measurement purposes. It is possible for a non-identical reference signal to produce a robust main correlation peak while producing negligible sidelobes when correlated with a synchronization code signal. The possibility of ranging errors due to the presence of sidelobes would then be reduced or eliminated.
It can be seen that there exists a need in the art of position determination utilizing broadcast TV signals for a method and system for generating reference signals that produce minimal sidelobes when correlated with the broadcast signals. However, it is a mathematical property of correlation processing that, when the sidelobes are reduced, the coupling to high frequency noise tends to increase. Depending on the characteristics of the signal, very noisy correlation output can also result in significant errors in pseudo-range measurements. Furthermore, there may also be coupling to low frequency disturbance signals. Thus, there also exists a need for a method and system for generating reference signals that, when correlated with the broadcast signals, minimize couplings to high frequency noise and low frequency disturbance signals in addition to producing minimal sidelobes and large main peaks. Accordingly, it would be desirable to provide a method and system for generating reference signals with such improved correlation characteristics. It would also be desirable to provide a method and system for accurate position determination utilizing broadcast TV signals by employing reference signals with improved correlation characteristics of minimal sidelobes, minimal couplings to high frequency noise and low frequency disturbance signals and maximizing the main correlation peak.